1. Field of the Invention
The present invention generally relates to a filter and a method for the design of a filter, specifically to a reception filter for a duplexer having an optimized bonding diagram and an improved isolation and stop band suppression, and a method for its design.
2. Description of the Related Art
Today, high-frequency filters are used in a wide range of applications, wherein different filter types produced in different technologies are used depending on application and requirement profile. In many cases, it is useful to produce filters in a particular technology optimized for filter applications and then to connect them with the rest of the circuit produced in another technology. One example for this are bulk acoustic wave filters (BAW filters) or thin film resonator filters (TFR filters), which are produced on a special substrate and are then connected to the surrounding circuit produced in another technology. When, however, two high-frequency components are to be connected, there is the problem that, when producing a connection, there is a discontinuity of the wave guiding structure. This results in parasitic effects which may affect the properties of the overall structure to a higher or lesser extent. Examples of such effects are a discontinuous change of the wave impedance, which may lead to reflections, and coupling and radiation effects which may produce couplings between remote circuit parts. In the following, the connection of a BAW filter or a TFR filter to a carrier material is discussed by way of example. It may, for example, be a reception filter implemented as thin film resonator filter in a duplexer for a wireless communication terminal. However, the description is transferable to all cases in which a high-frequency filter having a filter input and a filter output is connected to a carrier material with the aid of connecting wires.
If a filter having an input port and an output port is used in which the reference potentials of the input port and the output port are separated from each other, two conductive connections have to be established at each port. Both a signal line and a reference potential line have to be inserted between the actual filter and the carrier material comprising conductive structures at the input port and at the output port. A relevant criterion for the evaluation of the overall filter is the filter characteristic as it may be measured on the carrier material. This filter characteristic particularly includes the parasitic effects caused by the connecting wires. Two effects are dominant. On the one hand, it is unavoidable that the connecting wires represent a comparatively high inductance. This inductance may lead to detuning of the filter, whereby pass frequency ranges and/or stop frequency ranges are shifted. Furthermore, the inductances of the connecting wires change the adaptation of the filter, which shows in a change of the filter characteristic and an increase of losses. Besides, a coupling between the filter input and the filter output may be generated by the connecting wires. Such a coupling may result in the stop attenuation of the overall filter in a stop range being reduced, which means a deterioration of the filter characteristic.
Due to the above effects, it is necessary to optimize the connection between the actual filter and the carrier material, which, in most cases, is established by bond wires. Here, the reference potential connections are particularly important.
Previous approaches rely on optimizing the bond connection at the input port of the filter and at the output port of the filter separately. In particular, the ground connections at the input port and the output port are also optimized separately. The optimizing goal according to prior art is to achieve a minimum inductance of the ground connections and a minimum coupling between input and output ports. The low coupling involves both the bond wires for the reference potential (ground) and the bond wires for the active signal lines (input signal, output signal).
FIG. 3 shows a top view of a possible duplexer circuit. It is designated 410 in its entirety. It comprises a transmission filter 420 and a reception filter 422 mounted on a laminate carrier 424. The duplexer circuit 410 further comprises an antenna terminal 430 and a transmitter terminal 432 and a receiver terminal 434. Both the transmission filter 430 and the reception filter 422 are connected to the laminate carrier by bond wires. The transmission filter comprises an input pad 440, an output pad 442 and three reference potential pads 444, 446, 448. The input pad 440 and the output pad 442 are attached to opposite narrow sides of the transmission filter 420. The reference potential pads 444, 446, 448, however, are located along a broadside of the transmission filter 420. The input pad 440 of the transmission filter 420 is connected to a metallization on the laminate carrier 424 by a first bond wire 460 running across the first narrow side 470 of the transmission filter. The first bond wire 460 does not run exactly perpendicular to the first narrow side 470 of the transmission filter 420, but is slightly inclined with respect to the perpendicular. Similarly, the output pad 442 is connected to a metallization from the laminate carrier 424 via a second bond wire 480. The second bond wire 480 runs across the second narrow side 490, wherein it is slightly inclined in its direction with respect to a direction perpendicular to the narrow side 490. The reference potential pads 444, 446, 448 are also connected to corresponding metallizations from the laminate carrier 424 via bond wires 510, 512, 514. The bond wires 510, 512, 514 for the reference potential run substantially perpendicular to the broadside 520 of the transmission filter 420. This applies particularly to the bond wire 510 associated with the input and the bond wire 514 associated with the output. These two bond wires 510, 514 are both perpendicular to the broadside 520 of the transmission filter 420. The distance between the points at which the bond wires 510, 514 contact the laminate carrier 424 is equal to the distance between the points at which the bond wires 510, 514 contact the transmission filter.
The reception filter 422 is also connected to metallizations on the laminate carrier 424 via bond wires. All terminals of the reception filter are located along a first narrow side 540 and a second narrow side 542 opposite to the first narrow side 540. Along the first narrow side 540, the reception filter 442 comprises an input pad 550 and a reference potential pad 552 associated with the input. Similarly, the reception filter 422 comprises an output pad 560 and an associated reference potential pad 562 along the second narrow side 542. The reference potential pad 552 on the input side is part of a reference potential area on the reception filter 422. The same applies to the reference potential pad 562 on the output side. The reference potential areas on the input side and the output side on the reception filter 422 are isolated from each other. The input pad 550 on the reception filter is connected to a metallization for the input signal on the laminate carrier 542 via a bond wire 570. The reference potential pad 552 on the input side is connected to reference potential areas on the laminate carrier 424 via three parallel bond wires 572, 574, 576. All bond wires 570, 572, 574, 576 on the input side run perpendicular to the first narrow side 540. The bond wires 572, 574, 576 for the reference potential are each chosen as short as technically possible and practicable. The same also applies to the bond wires 580, 582, 584, 586 on the output side. Thus it is to be noted that, in a conventional prior art duplexer circuit 410, the bond wires of the reception filter 422 on the input side and the output side run perpendicular to the first narrow side 540 and the second narrow side 542 of the filter, and that the reference potential wires 572, 574, 576, 582, 584, 586 are chosen as short as possible.
FIG. 5 shows an equivalent circuit diagram of a reception filter having a filter structure according to prior art. The equivalent circuit diagram describes a reception filter structure such as it is present in a prior art duplexer circuit 410. The entire equivalent circuit diagram is designated 610. The heart of the filter structure is the equivalent circuit diagram 620 of the reception filter. The equivalent circuit diagram 620 comprises four terminals. The input of the reception filter is formed by the input terminal IN with which there is associated a reference potential terminal GND_IN on the input side. The filter output is formed by an output terminal OUT and a reference potential terminal GND_OUT associated therewith.
The effect of the bond wire connecting the input pad 550 of the reception filter 422 to a metallization on the laminate carrier 424 is described by an inductance 630 whose magnitude may be estimated at 0.4 nH. Similarly, the bond wire 580 connecting the output pad 560 of the reception filter 422 to a conductor structure on the laminate carrier 424 may be modulated by an inductance 632 whose magnitude may also be estimated at 0.4 nH. The inductances 630 and 632, respectively, are connected in series to the input terminal IN and the output terminal OUT, respectively, of the reception filter model 620. Furthermore, one has to take into account the inductance of the bond wires connecting the reference potential area 552 of the reception filter 422 on the input side to a reference potential area on the laminate carrier 424. As these bond wires 572, 574, 576 are very short and as three bond wires are connected in parallel, the inductance is less than that of the bond wire 570 for the input signal. In an equivalent circuit diagram, the input side connection of the filter to the reference potential on the laminate carrier may be modulated by an inductance 640 which may be estimated at 0.25 nH. The same applies to the output side connection of the reception filter 422 to a reference potential area on the laminate carrier 424. Again, there will be an inductance 642 of about 0.25 nH. The distance of the connection points at which the bond wires 572, 574, 576, 582, 584, 586 for the reference potential on the input side and/or on the output side contact a metallization on the laminate carrier 424 is comparatively large (larger than the larger dimension of the filter). Therefore, an inductive coupling between the corresponding connection points on the input side and the output side may be considered as negligible. Correspondingly, a coupling inductance 650 on a metallization area (reference potential area) of the laminate carrier is to be regarded as very large. In a simplified modeling, the value of the coupling inductance 650 approaches infinity, which means that the impedance of the coupling inductance is very high. A large coupling inductance therefore means little coupling. Furthermore, the filter structure includes vias. They contribute an inductance of about 50 pH. The corresponding inductances are designated 660 and 662, respectively. Thus, the result is an equivalent circuit diagram of the filter structure in which two inductances 640, 642 are connected in series to the reference potential terminal GND_IN on the input side and the reference potential terminal GND_OUT on the output side, respectively. The inductances are each connected to the reference potential via the via inductances 660, 662. There is a coupling inductance 650 between the two branches whose value is assumed to be infinitely large in the present modeling, corresponding to a negligible coupling.
EP 1202455-A2 describes a variety of possibilities how to optimize the characteristic of a filter mounted to a carrier material by means of bond wires. The reference essentially follows the above rules. A fundamental goal of an embodiment according to the reference is to reduce the coupling between input and output ports. For this., the bond pads associated with the input port and the bond pads associated with the output port are arranged at a maximum distance from each other. Thus, the current loops of the filter input and the filter output are as distant as possible from each other. The area of the current loops at the filter input and the filter output is to be minimized according to the reference. This may be achieved by the bond pads associated with a port (i.e. active signal terminal and the associated reference potential terminal) being as close to each other as possible both on the actual filter substrate and on the carrier material. Furthermore, the reference teaches to use the shortest bond wires possible. In order to shorten the bond wires, it also helps to design the filter substrate as thin as possible. Finally, the reference recommends to design bond wires associated with different ports as orthogonal as possible, because this also helps to reduce the coupling. The cited reference also teaches to optimize the individual ground connections of the filter separately, whereby the inductances of the ground connections are typically minimized. It is a disadvantage of such an approach that the stop band suppression of a filter is thus narrow-band. Under certain circumstances, minimizing the bond wire inductances may further require additional inductances, for example for adaptation, which have to be realized on the carrier material. The result is that a larger housing is required than would be necessary without the use of external inductances.